linux-kernel - Re: [PATCH v2] KVM: selftests: Fix target thread to be migrated in rseq

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Date:   Sat, 16 Jul 2022 21:48:13 +0000
From:   oliver.upton@...ux.dev
To:     "Gavin Shan" <gshan@...hat.com>, kvmarm@...ts.cs.columbia.edu
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, seanjc@...gle.com,
        pbonzini@...hat.com, maz@...nel.org, shuah@...nel.org,
        shan.gavin@...il.com
Subject: Re: [PATCH v2] KVM: selftests: Fix target thread to be migrated
 in rseq_test

Hi Gavin,

Thanks for cleaning this up.

July 16, 2022 7:45 AM, "Gavin Shan" <gshan@...hat.com> wrote:
> In rseq_test, there are two threads, which are thread group leader
> and migration worker. The migration worker relies on sched_setaffinity()
> to force migration on the thread group leader.

It may be clearer to describe it as a vCPU thread and a migration worker
thread. The meat of this test is to catch a regression in KVM.

> Unfortunately, we have

s/we have/the test has the/

> wrong parameter (0) passed to sched_getaffinity().

wrong PID

> It's actually
> forcing migration on the migration worker instead of the thread group
> leader.

What's missing is _why_ the migration worker is getting moved around by
the call. Perhaps instead it is better to state what a PID of 0 implies,
for those of us who haven't read their manpages in a while ;-)

> It also means migration can happen on the thread group leader
> at any time, which eventually leads to failure as the following logs
> show.
> 
> host# uname -r
> 5.19.0-rc6-gavin+
> host# # cat /proc/cpuinfo | grep processor | tail -n 1
> processor : 223
> host# pwd
> /home/gavin/sandbox/linux.main/tools/testing/selftests/kvm
> host# for i in `seq 1 100`; \
> do echo "--------> $i"; ./rseq_test; done
> --------> 1
> --------> 2
> --------> 3
> --------> 4
> --------> 5
> --------> 6
> ==== Test Assertion Failure ====
> rseq_test.c:265: rseq_cpu == cpu
> pid=3925 tid=3925 errno=4 - Interrupted system call
> 1 0x0000000000401963: main at rseq_test.c:265 (discriminator 2)
> 2 0x0000ffffb044affb: ?? ??:0
> 3 0x0000ffffb044b0c7: ?? ??:0
> 4 0x0000000000401a6f: _start at ??:?
> rseq CPU = 4, sched CPU = 27
> 
> This fixes the issue by passing correct parameter, tid of the group
> thread leader, to sched_setaffinity().

Kernel commit messages should have an imperative tone:

Fix the issue by ...

> Fixes: 61e52f1630f5 ("KVM: selftests: Add a test for KVM_RUN+rseq to detect task migration bugs")
> Signed-off-by: Gavin Shan <gshan@...hat.com>

With the comments on the commit message addressed:

Reviewed-by: Oliver Upton <oliver.upton@...ux.dev>